I have a 2000 1.6K supersport, It revs really sweetly but does need keeping in the power to really move. I believe that the 1.8 has more torque, but does not rev as well. I love to thrash the T*t* off my car so am quite happy with the 1.6.
It really depends what you are after. I do not think that you will have that many reliability problems tuning either engines to 150BHP ( I believe that the general consensus is that around 190 bhp is the limit before major internal surgery is needed on either engine), I am looking to progress to around 175BHP through 4-2-1 exhaust, throttle bodies, emerald cams and programable chip (my car was originally a roadsport which I upgraded to supersport, something I now think was a bit of a dead end), I believe this will sort the torque out, and increase peak power.
I use my car largely on the road, for quick moorland blats and quite a few longer trips, the only time I would prefer the larger engine is on main roads and motorways. planting the throttle in fifth at 60mph (standard five speed box, due for mods) just results in farty noises and a postal invitation to change down to third, This situation is not really what a Caterham is meant for, of course, I am biased, over to you.
Regards Mark